For the production of a functional film, especially a crystalline semiconductor film, an appropriate method has been employed in view of the required characteristics, use purpose or the like for the film to be obtained.
That is, there have been proposed various methods using vacuum evaporation technique, thermal induced chemical vapor deposition technique, plasma chemical vapor deposition technique, reactive sputtering technique, ion plating technique and light induced chemical vapor deposition technique.
Among those methods, the method of thermal induced chemical vapor deposition (hereinafter referred to as "CVD method") had once been frequently used in various applications. However, such methods are not usually employed for the reason that, besides requiring an elevated temperature, a practically usable film cannot be obtained as expected.
On the other hand, the plasma chemical vapor deposition method (hereinafter referred to as "plasma CVD method") has been generally evaluated as being the most preferred and is currently used to prepare a deposited film on a commercial basis.
Now, although the plasma CVD method is widely used nowadays as above mentioned, that method is problematical due to the fact that it is practiced under elevated temperature conditions and other problems are associated with the apparatus to be used.
Regarding the former problems, because the plasma CVD method is practiced while maintaining substrate at an elevated temperature, firstly the kind of a substrate to be used is limited to one that does not contain a material such as a heavy metal which can migrate and cause changes in the characteristics of a deposited film to be formed and secondly, its thickness is likely to be varied, whereby the resulting film lacks uniformity of thickness and homogeneity of the composition, which may itself also cause changes in the characteristics of the film to be formed.
Regarding the latter problems, the operating conditions employed with the plasma CVD method are much more complicated than the known CVD method, and are extremely difficult to be generalized.
That is, there already exist a number of variations even in the correlated parameters of substrate temperature, the amount and the flow rate of gases to be introduced, the pressure, the high frequency power for forming a film the structure of the electrodes, the structure of the reaction chamber, the exhaust rate, the plasma generation system, etc. Under these circumstances, in order to prepare a desirable functional deposited film for electron devices, it is required to choose precise parameters from a great number of varied parameters.
There sometimes occurs a serious problem that because of the precisely chosen parameters, the plasma may attain an unstable state which often imparts unexpected troublesome effects to functional deposited film to be formed.
In addition, in the case of desiring to form a crystalline functional deposited film, stably producing such film in accordance with the plasma CVD method is considered to be difficult since the related conditions to make a film to be crystalline are extremely limited under the plasma CVD method.
Further, in recent years, the public attention has been forcused on functional crystalline films such as functional epitaxial films and functional polycrystalline films constituted with atoms of Groups IIB and VIA or atoms of Groups IIIA and VA of the Periodic Table because of their wide usefulness.
For their production, there have been proposed various methods which can be classified into two categories of vapor phase epitaxy and liquid phase epitaxy.
The liquid phase epitaxy is a method of depositing a semiconductor crystal on a selected substrate by resolving a raw material for semiconductor in a metallic solvent in liquid state at elevated temperature until the raw material becomes supersaturated, and cooling the solution.
For this method, there is an advantage that such a crystalline product as having a relevant completeness can be obtained since it is prepared in the state of being most closest to thermal equilibrium among various epitaxy techniques. However, there are unavoidable disadvantages, particularly, in the case of preparing optical devices for which an epitaxial layer having a thin and uniform thickness is required to be used. That is, the yield in its preparation is unsatisfactory and undesirable influences occur because of poor mass productivity and unstable surface state for the film to be formed.
In view of the above, the liquid phase epitaxy is nowadays seldom used.
On the other hand, as for the vapor phase epitaxy, it has been often tried to practice it using physical methods such as vacuum evaporation and reactive sputtering, and chemical methods such as hydrogen reduction and thermal cracking using organometallic compounds or metal hydrides. Among these methods, the molecular beam epitaxy, a kind of the vacuum evaporation deposition, is a dry process practiced under ultra-high vacuum. Therefore, there are advantages for said epitaxy in that it is possible to make a product highly purified and to deposit a film at low temperature, and a relatively plane deposited film can be obtained. However, even for such epitaxy, there are disadvantages which remain unresolved in that surface defect density is large, a practically applicable method for controlling the directivity of the molecular beam is yet to be practically developed, production of a large square deposited film is difficult, and mass productivity is insufficient. In addition these disadvantages, an apparatus for practicing such epitaxy is too costly.
In view of the above, the molecular beam epitaxy is not yet practiced on an industrial scale.
As for said hydrogen reduction method and thermal cracking method, they are generally called halide CVD method, hydride CVD method and MO-CVD method. These methods are generally evaluated as being worthy of being discussed since there are advantages that a film forming apparatus therefor can be relatively easily manufactured and, highly purified metal halides, metal hydrides and organometals which are used as raw materials in such methods are generally available.
However, under these methods, since the temperature of a substrate is required to be high enough to allow the occurrence of the reduction reaction or thermal cracking, there is a limit for the kind of the substrate which can be used. In addition, in the case where the raw material is not sufficiently decomposed, contaminations with the impurities such as carbon atoms, halogen atoms are apt to occur and as a result, it becomes difficult to control the degree of doping.
Under this circumstance, even if a desirable functional crystalline film should be fortunately produced, the functional crystalline film product will become costly for the reasons that a heavy investment is necessitated to set up a particularly appropriate apparatus therefor.
In this regard, for mass-producing a desirable functional crystalline deposited film, particularly, a functional silicon-containing or germanium-containing epitaxial film, it is desired to modify or change the currently known methods to such that makes one possible to effectively form such film.
In fact, there is now an increased demand for providing a method that makes it possible to practice the process at lower temperature and at a high film forming rate in a simple apparatus to mass-produce a desirable functional epitaxial film applicable in any electron devices which has satisfactory uniformity and has practically applicable characteristics.